Transition metal chalcogenides have layered structures similar to that of graphite. In particular, MoS2 is drawing attentions as a semiconductor active layer for a transistor that can replace graphene and as a hydrogen evolution reaction catalyst that can replace platinum. Also, MoS2 is being studied as an electrode material of a lithium-ion battery because it has a layered structure allowing easy intercalation and extraction of lithium ions [Chhowalla, M. et al., Nature Chemistry 2013, 5, 263-275].
A bulk MoS2 single crystal has an indirect band gap of 1.3 eV unlike graphene and has a superior mobility of 50-200 cm2/Vs at room temperature. Since it has a direct band gap of 1.8 eV when the thickness decreases to the scale of a single atomic layer, researches are actively carried out to make it into a thin film for use as an active layer of a transistor [Wang, Q. H. et al., Nature Nanotechnology 2012, 7, 699-712]. Most recently, it has been known that a thin film prepared from monolayer, bilayer or multilayer MoS2 has a mobility close to that of bulk MoS2.
As methods for manufacturing MoS2 thin films, exfoliation of detaching an atomic layer from a MoS2 single crystal and chemical vapor deposition of depositing MoS2 on, e.g., a substrate at high temperature using Mo (or MoO3) and sulfur as precursors are being studied. However, these methods are inapplicable to large-scale production processes (particularly, semiconductor processes). In addition, the chemical vapor deposition method is limited in that it is difficult to control the number of atomic layers.
MIT's Wang, H. et al. reported in IEEE Tech. Dig. IEDM, 88-91 (2012) (non-patent document 1) that a MoS2 atomic layer for a transistor prepared at 650° C. using MoO3 and S (elemental sulfur) as precursors has a mobility of approximately 190 cm2/Vs. However, because the precursors used are solids and have very low vapor pressures, the chemical vapor deposition method is not applicable to large-scale production due to contamination of equipment such as a vacuum chamber.
Although the atomic layer deposition method (ALD) of growing a thin film trough chemical adsorption of precursors is the best suited method for growing atomic layers, it is not utilized for the growth of thin film or monolayer of layered transition metal sulfides such as MoS2. In the atomic layer deposition method, atomic layers are formed from chemical adsorption between the precursors and surface functional groups. A thin film is formed by chemically adsorbing two different precursors alternatingly. In general, the atomic layer deposition method using two precursors consists of cycles each consisting of adsorption of a first precursor and purging and adsorption of a second precursor and purging. The thickness of the thin film can be controlled in the scale of atomic layers by controlling the number of cycle.
Although it is expected that a thin film of MoS2 can be formed by the atomic layer deposition method because it consists of only two elements of Mo (molybdenum) and S (sulfur), the growth of MoS2 thin film by the atomic layer deposition method has not been reported yet due to the absence of a suitable precursor. In particular, although MoF6, MoCl6, Mo(CO)6, etc. are known as Mo precursors, no suitable sulfur precursor has been designed. Although use of H2S as the sulfur precursor may be considered as H2O is used as an oxygen precursor, the H2S gas is not applicable to a large-scale production process due to its toxicity, corrosiveness and explosiveness.